1. Field of the Invention
The present invention is directed to a method for processing a radiation image sequence that is composed of a number of successively acquired radiation images acquired with a dynamic image converter, particularly a solid-state image detector, as well as to a medical examination apparatus operating according to such a method.
2. Description of the Prior Art
Radiation image exposures, particularly in the fluoroscopic mode, are utilized for the observation of dynamic events and for diagnosis. Low-power radiation is employed for this purpose. The voltage and the current of the radiation source, i.e. of the X-ray tube, must be set with the required contrast and the lowest possible dose for patient and examining person taken into consideration. The unavoidable presence of noise in the framework of the image acquisition plays an important part due to the relatively low radiation dose in the fluoroscopic mode. In the case of solid-state image detectors that are being increasingly employed, two noise sources exist, namely quantum noise and internal detector noise. Similar problems also occur in the digital cinematographic mode wherein images are registered with a higher dose over a shorter time span.
In order to compensate the influences of the noise as much as possible with respect to adequate image contrast and signal-to-noise ratio, it is known to subject the acquired images to a filtering; but this does not always lead to satisfactory results.
German PS 26 55 525 discloses the resolution of images of a sequence into different spectral components, which are merged preceding the integrating image memory, so that all spatial frequencies are temporally filtered in the same way.
German OS 39 31 934 discloses acquiring a packet of images with different focusing planes, with each image being subjected to a Fourier transformation. Subsequently, a selection of specific spatial frequencies occurs for each transformed image. These selected spatial frequency components are then added and subsequently back-transformed. This addition practically represents an overall time integration of the image packet.
German OS 198 49 090 and U.S. Pat. No. 5,880,767 disclose methods for processing input images wherein different spatial frequency components are selected from an image and added after weighting.
Spiesberger, W. et al., “Processing of Medical Image Sequences”, in Huang, T. S., Image Sequence Analysis, Springer Verlag, 1981, pages 381 through 426, discloses a method for level-dependent temporal filtering, wherein a motion-dependent temporal averaging (for example, recursive filter with motion detector) ensues on the basis of relative levels between the individual images of a sequence at a specific image location (picture element).